The present invention relates to a sintered body of a carbonitride alloy with titanium as main component which has improved properties particularly when used as cutting tool material in light finishing cutting operations at high cutting speeds. More particularly, the present invention provides a carbo-nitride based hard phase of specific chemical composition with an extremely solution hardened Co-based binder phase. The binder phase has properties similar to the binder phase of WCxe2x80x94Co-based materials except that it has been possible to increase the solution hardening beyond the point where eta-phase normally would appear.
Titanium-based carbonitride alloys, so called cermets, are produced by powder metallurgical methods and comprise carbonitride hard constituents embedded in a metallic binder phase. The hard constituent grains generally have a complex structure with a core surrounded by a rim of a different composition. In addition to titanium and group VIa elements, both molybdenum and tungsten are usually added to facilitate wetting between binder and hard constituents and to strengthen the binder by means of solution hardening. Group IVa and/or Va elements, e.g. Zr, Hf, V, Nb, and Ta, are also added in all commercial alloys available today. The carbonitride forming elements are usually added as carbides, nitrides and/or carbonitrides. Historically, the binder phase in cermets has been nickel, most likely because Ti has a high solubility in Ni to facilitate sufficient wetting to obtain a low porosity level. During the 1970s a solid solution binder of cobalt and nickel was introduced. Most likely, this was made possible by improved raw material quality, in particular a lower impurity level of oxygen. Today all commercial alloys contain 3-25 wt % of a solid solution binder with relative proportions Co/(Co+Ni) in the range 50-75 at %.
Cermets are well established as insert material in the metal cutting industry today. Compared to WCxe2x80x94Co based materials they have excellent chemical stability when in contact with hot steel, even when uncoated, but have substantially lower strength. This makes them most suited for finishing operations, which generally are characterized by limited mechanical loads on the cutting edge and a high surface finish requirement on the finished component. Unfortunately, cermets suffer from an unpredictable wear behavior. In a worst case, tool failure is caused by bulk fracture which may lead to severe damage of work piece as well as the tool holder and cutting machine. More often, tool failure is caused by small edge line fractures, which abruptly change the surface finish or dimensions obtained. Common to both types of failure is that they are stochastic in nature and occur without previous warning. For these reasons cermets have a relatively low market share, especially in modern highly automated production which relies on a high degree of predictability to avoid costly production stops.
One way to improve predictability, within the intended application area, would be to increase the toughness of the material and work with a larger safety margin. However, so far this has not been possible without simultaneously reducing the wear and deformation resistance of the material to a degree which substantially lowers productivity.
It is an object of the present invention to solve the problem described above and others. It is indeed possible to design and produce a material with substantially improved toughness while maintaining deformation and wear resistance on the same level as conventional cermets. This has been achieved by working with the alloy system Tixe2x80x94Taxe2x80x94Wxe2x80x94Cxe2x80x94Nxe2x80x94Co. Within this system a set of constraints has been found rendering optimum properties for the intended application area. As so often is the case, the solution is not one single major change but a combination of the following precise requirements which together give the desired properties.
According to one aspect of the present invention, there is provided a titanium based carbonitride alloy comprising Ti, Ta, W, C, N and Co, for light finishing operations at high cutting speeds having a binder comprising 3 to  less than 9 at % Co with only impurity levels of Ni and Fe.